These studies address the interaction of signals from rods and from cones within the human visual system. The objectives are to collect basic psychophysical data on the temporal and spatial characteristics of interaction, on the sources of the interacting signals, and on the algebraic combination rules of interaction. The methods rely on keeping rod excitation constant - either over time or over space or both - while varying cone excitation and measuring the sensitivity of the rod system. The time course of interaction is measured to identify the properties of transient and sustained interactions. The spatial characteristics of interaction are measured by means of backgrounds of various shapes and sizes in order to identify the properties of the mediating pathways. The combination rules of interaction are studied by relating the magnitude of desensitization of the cone system to the magnitude of interaction. This may also provide a quick test for interaction that could aid other researchers. These experiments also allow us to measure the role of separate cone mechanisms in interaction. Dichoptic experiments allow us to study interaction as it may occur beyond the retina. Stationary and rotating windmill-shaped backgrounds permit a different test of the dynamics of interaction and assess the correspondence of interaction in humans to physiological results with other animals. The results will advance our knowledge of the human visual system, especially the shared pathways of rod and cone signals. They will also guide the physiologically study of interaction and the application of physiological data to the human visual system.